1. Field of the Invention
The present invention relates to an apparatus which can measure a weak magnetic field with a higher accuracy, reliability, and stability by using a magneto-resistive device. the magneto-resistive device in composed of a ceramic superconductive film having weak coupled grain boundaries of higher sensitivity to a weak magnetic field than conventional magneto-resistive devices.
2. Description of the Prior Art
Magneto-resistive devices using semiconductors or magnetic materials are conventionally used for measuring magnetism. These magneto-resistive devices show a maximal sensitivity of about 10.sup.-3 gauss, and the sensitive property is represented by a quadratic curve to a magnetic field as shown in FIG. 1. In such a magneto-resistive device, its sensitivity becomes low when an applied magnetic field is weak.
There is a SQUID :magnetic flux meter using a superconductive sensor which is sensitive to a very weak magnetic field. The squid however, suffers from problems because it must be chilled to an extremely low temperature, and requires troublesome measuring operations.
There is proposed a highly sensitive magnetic sensor utilizing the weak coupling property of the grain boundary of the ceramic superconductor having a high critical temperature in the Japanese Patent Application No. 233369/1987. As shown in FIG. 3, the magnetic sensor of the above kind is simply constructed by a ceramic superconductor 1, current electrodes 2 and 3, and voltage electrodes 4 and 5, and can induce electrical resistance to an applied weak magnetic field as shown in FIG. 2. The electrical resistance is sharply increased corresponding to increase of the magnetic field strength.
The operation of the superconductive magneto-resistive device was examined in detail under the condition that a predetermined current was fed to the device, and that a magnetic field was applied thereto in variable strengths. As a result, it is found that it shows a hysteresis property as shown in FIG. 5.
The superconductive magneto-resistive device having the above mentioned hysteresis feature causes significant errors when detecting or measuring a weak magnetic field.
In the Japanese Patent Application No. 193018/1987, there was proposed a driving system of a superconductive magneto-resistive device which can always keep a predetermined output for an applied magnetic field strength. The proposed driving system has however a troublesome problem which is, that in order to lower a bias current to the device, once the current must be decreased to zero, and thereafter it is necessary to increase the current to set to a predetermined value, so that the device can be operated by a constant characteristic curve.